OK, thanks for your reply.
I probably miss something about the inner workings of the Android operating system, sorry about that.

Nevertheless, let me share with you the problem I’ve faced today:

I switched off my phone in the evening, yesterday.
Then switched back on after a few minutes.
DejaOffice haven’t fired the notification timed for the next morning (today, 08:00), only after I had started DejaOffice at 09:10 – so it has completely missed the alarm.

Maybe DejaOffice is unable to start its necessary processes after a phone reboot to update the alarm system of Android?